1. Field of the Invention
The present invention pertains to an installation for imaging by nuclear magnetic resonance. The invention pertains especially to means which are used to shield instruments placed in the environment of the device that produces a magnetic field from the effect of the said magnetic field. This is the case, especially, for cathode screen type monitors used in the operating consoles of devices for imaging by nuclear magnetic resonance.
2. Description of the Prior Art
A nuclear magnetic resonance imaging device or an NMR device especially comprises a magnet which produces an intense, constant magnetic field in the device to which the body of a patient to be examined is subjected. Such a device is for example depicted in ELECTROMEDICA, volume 51, n.degree.2, 1983, M. MORNEBURG: "GESICHTSPUNKTE BEI DER STANDORT SUCHE FUR EIN MAGNETOM", page 67-72.
The NMR device is put into operation by an operator or operators who act at an operating console to define the various parameters and control the various operations needed to obtain the desired image. For this purpose, an operating console generally has two monitors: a first monitor called an image monitor designed to display the image obtained and a second monitor called a dialogue monitor which, in particular, enables an operator to adjust the different parameters. The use of the dialogue monitor is made much easier when it is placed beneath a tactile screen. But it must be noted that the use of a tactile screen is not really worthwhile unless the dialogue monitor is itself a color monitor.
In general, the MRI device itself is placed in a first room which forms a Faraday cage. The operating console is placed in an adjoining room, and the wall between the two rooms has a glass window so that the operators or doctors do not lose sight of the patient throughout the time when he is placed in the MRI device, so that they can quickly be at his side if there is any hitch. For this reason, and also to reduce the area of the premises needed, it is sought to place the operating console at a relatively small distance from the MRI device, for example, at less than 10 meters.
The problem that arises here relates to the use of the the monitors when they are subjected to the magnetic field produced by the MRI device.
For, when a monitor is set up in a magnetic field, it undergoes disturbances which make it unusable. For example, with a black-and-white monitor placed in a three-Gauss magnetic field, it is impossible to use a tactile screen properly. With a color screen, the constraint is even greater and the color screen is practically unusable when it is in a field of more than than 0.5 gauss.
The intense field produced by the MRI instrument can reach several thousands of Gauss in the instrument itself, so that in the control next to the examination room, the leakage field of the magnet, even at 10 meters, is still far greater than the values referred to above. Consequently, it is noted that in the prior art, only black-and-white monitors can be used with the operating console at less than 10 meters from the MRI instrument, even then provided that these monitors are contained in boxes forming shields. The use of a shield placed, for example, around a monitor, not only makes it impossible to use a color monitor at less than 10 meters from the NMR device but also has the disadvantage of requiring a setting of the black-and-white instruments. For despite the use of shielding, this residual spurious field makes it necessary to adjust the deflection coils proper to the monitor in order to make it usable. This disadvantage is a particularly big one because the adjustment cannot be done unless the black-and-white monitor is taken out of its shield and the result of the adjustment is not visible unless the monitor is again reintroduced into its shielding box. Thus, several successive adjusting operations are needed before the optimum setting is obtained.
It is known by the EP-A No. 0 039 502 application a device for correcting the field effects on color monitor. Meanwhile this device when it is set lead to the unsettlement of a similar device placed on a neighbouring monitor. As a result the entire correction is long to be achieved for the two monitors.
3. Summary of the Invention
A particular object of the present invention is to enable to the use of at least one black-and-white or color monitor in a nuclear magnetic resonance imaging insulation at a distance from the device which is far smaller than in the prior art for the same field intensity, and to enable the use of the monitor or monitors in a far simpler and surer way than in the prior art. It must be observed that, in substantially the same conditions, the invention can be applied to the shielding of other instruments from the magnetic field, for example, magnetic tapes for which the use and storage near an MRI device may raise problems.
According to the invention, a nuclear magnetic resonance imaging installation comprises a device to produce an intense magnetic field, the said device being at a given distance from at least one instrument the operation of which may be disturbed by the intense magnetic field, the said installation comprising means to produce a second magnetic field in a direction substantially opposite to that of the intense magnetic field, so that the second magnetic field tends to cancel the first intense magnetic field at the level of the instrument.